Before an elite team of U.S. Navy SEALs executed a daring raid that took down Osama bin Laden, the commandos were able to silently sneak up on their elusive target thanks to what aviation analysts said were top secret, never-before-seen stealth-modified helicopters.

In the course of the operation that cost the al Qaeda leader his life, one of the two Blackhawk helicopters that carried the SEALs into bin Laden's Pakistani compound grazed one of the compound's wall and was forced to make a hard landing. With the chopper inoperable, at the end of the mission the SEALs destroyed it with explosives.

But photos of what survived the explosion -- the tail section of the craft with curious modifications -- has sent military analysts buzzing about a stealth helicopter program that was only rumored to exist. From a modified tail boom to a noise reducing covering on the rear rotors and a special high-tech material similar to that used in stealth fighters, former Department of Defense official and vice president of the Lexington Institute Dan Goure said the bird is like nothing he's ever seen before.

MAkes sense they have been doing this to warships for years and of course the stealth bomber. It makes them impervious to radar.

There's one thing that would make a "stealth helicopter" stick out on radar that they can't fix: Rotor blades turning at hundreds of RPM. Those blades turning really stand out on radar.

Also, I'm not sure how much noise mitigation they've come up with over the years but from personal experience, a person can hear a helicopter coming from pretty far out depending on wind direction/velocity, altitude of the aircraft, etc.

Still, it's a cool concept... I just don't think that the physics of how a helicopter flies supports it.

This really sucks, because the parts that are left are in the hands of Pakistan, and could be sent to the Chinese. If our leaders had any brains they'd demand the parts back or the Pakistani's get no more aid.

"In episode 4 in the first series of the American television series King of the Hill, Dale Gribble claims a noise to be that of "one of those stealth helicopters with computerised noise- cancellation capability... they're still workin' the 'chings' out." When Bill Dauterive asks him how he knows about stealth helicopters, Dale replies: "alt.conspiracy.black.helicopters"."

There's one thing that would make a "stealth helicopter" stick out on radar that they can't fix: Rotor blades turning at hundreds of RPM. Those blades turning really stand out on radar.

Also, I'm not sure how much noise mitigation they've come up with over the years but from personal experience, a person can hear a helicopter coming from pretty far out depending on wind direction/velocity, altitude of the aircraft, etc.

Still, it's a cool concept... I just don't think that the physics of how a helicopter flies supports it.

That's exactly what they are talking about. It's not just the material it's made with. It's the functionality. Take in mind, the story said, the helicopter "grazed" the compound and it took them getting that close to be detected. (If indeed it was shot down by enemy forces).

Quote:

The stricken aircraft, which commandos blew up at the scene, appears to be a long-rumored stealth helicopter, NBC News reported; a Black Hawk modified to reduce rotor noise and make it more difficult to detect by radar.

Pentagon officials have declined to comment.

Aviation Week reported that the chopper's tail appears to be highly modified compared to a standard H-60 Black Hawk. The report noted that stealth features on a helicopter usually aim to dampen rotor noise and reduce infrared signals.

Noise reduction can be accomplished by adding blades to the rotors and changing the way the pilot flies the chopper, such as flying in a manner to reduce the rotor's rpm, the report said.

As for reducing the likelihood of the aircraft chopper giving off infrared signals, Aviation Week pointed to an earlier helicopter prototype that had a complex exhaust system and fresh air ejectors to lessen the aircraft's heat signals

This really sucks, because the parts that are left are in the hands of Pakistan, and could be sent to the Chinese. If our leaders had any brains they'd demand the parts back or the Pakistani's get no more aid.

I think the Chinese are still trying to reverse engineer our stealth plane that got shot down 20 years ago.

There's one thing that would make a "stealth helicopter" stick out on radar that they can't fix: Rotor blades turning at hundreds of RPM. Those blades turning really stand out on radar.

Also, I'm not sure how much noise mitigation they've come up with over the years but from personal experience, a person can hear a helicopter coming from pretty far out depending on wind direction/velocity, altitude of the aircraft, etc.

Still, it's a cool concept... I just don't think that the physics of how a helicopter flies supports it.

Coming from a former Navy Helo crewman who was able to work with Navy SEALs on Combat SAR training...

You'd be surprised what we can do do minimize the sound signature on approach...

Now in a hover that's a differnt story... you are obviously most vulnerable and noisy when in a hover.

The stricken aircraft, which commandos blew up at the scene, appears to be a long-rumored stealth helicopter, NBC News reported; a Black Hawk modified to reduce rotor noise and make it more difficult to detect by radar.

Pentagon officials have declined to comment.

Aviation Week reported that the chopper's tail appears to be highly modified compared to a standard H-60 Black Hawk. The report noted that stealth features on a helicopter usually aim to dampen rotor noise and reduce infrared signals.

Noise reduction can be accomplished by adding blades to the rotors and changing the way the pilot flies the chopper, such as flying in a manner to reduce the rotor's rpm, the report said.

As for reducing the likelihood of the aircraft chopper giving off infrared signals, Aviation Week pointed to an earlier helicopter prototype that had a complex exhaust system and fresh air ejectors to lessen the aircraft's heat signals

What some may not realize is the H-60 has a constant rotor speed. it doesn't spin faster to accelerate or slower to decelerate. Now under heavy load the rotors do slow down but due to the load not due to any direct input from the pilot. The invers is also true, under light load the rotor will spin a little faster but that's still a function of decreased load to the rotor head.

So, it's all based on the angle of attack of the blades and the rotor head which on an H-60 is a fully articulated rotor head...

the rotor head can pivot in all directions and the blades twist on their longitudinal axis...

this combined with the angle of the helo results in accelleration or deceleration,

The angle of the blades will result in Climb or decent.

but the pilots are trained to know what angles are quieter than others to help with the element of surprise....

edit: Note in the two pics below the rotor RPMs are nearly identical.

this image is a helo accellerating forward and climbing... This pic gets my blood pumping..